1. Field of the Invention
This invention relates to a relatively compact encoder apparatus for precisely measuring linear motion and angular rotation values, and more particularly, to a new and improved laser Doppler encoder that incrementally measures linear and angular displacements by transmitting a laser beam through a self-aligning, multiple reflective optical path.
2. Background of the Invention
It has becomes necessary in connection with manufacturing in high technology industries to very precisely control the motion of tools, automated robots, and control and metrology instrumentation. For example, such precise control of tools has become almost a necessity in lithography, chip manufacturing, and X-ray micromachining for microsensors. In addition, the study of materials and new science with X-rays, such as in the fields of DNA, human genome and structural biology, require high precision motion and detector systems which should function with tolerances in atomic and sub-atomic dimensions (usually a few Angstrom-nanometer levels). In this regard, typical research tools include high-resolution X-ray monochromators (for precise energy discrimination typically one part in a million or better), large-field scanning microscopes, profilometers, and X-ray microscopes.
Linear and angular encoders are digital devices that are utilized in these technologies and instrumentation to determine precisely linear motions and angular rotations of components. The encoders are currently available with resolutions limited to several tenths of a manometer and a few nano-radians in linear and angular resolution, respectively, and are generally limited to linear measurements in about the millimeter range and angular measurements in about a sub-degree range.
In connection with laser Doppler angular encoders (LDAE), no commercially available angular encoder provides a sub-nano-radian resolution in the 5-8 degree measuring range (a laboratory setup encoder has been reported that has achieved a resolution of a few nano-radians, but this is a large setup (about 610 mm.times.1220 mm) that is based on a polarization-encoded Michelson interferometer principle such that the measuring speed is undesirably slow). In the much larger 10-20 nano-radian (nrad) resolution range, an angular sensor currently on the market (Applied Geomechanics, Model-520) has a 10-nrad resolution, but this sensor only covers a measuring range of less than 0.01 degree with a very long measuring settling time (0.1-30 seconds). In the still larger 20-100 nrad resolution range, laser-based machine tool calibration systems exist, such as Hewlett Packard HP-5527B and Zygo ZMI-1000. From a few degrees up to 20-degree angular measuring range, these units provide a 20-100 nrad angular resolution, which is about two orders of magnitude less precise than what needs to be obtained in such a system. In the field of grating-based optical encoders, a model ROD-800 from Heidenhain is available. This encoder has a 250 nrad resolution with a 360-degree measuring range.
As can be appreciated, these types of available encoders are not acceptable for applications where very precise angular rotations measurements (for example, sub-nano-radian angular resolution) are required over at least a 5-8 degree measuring range. What is needed is a relatively compact encoder that provides an adequate, very precise resolution in an acceptable measuring range while providing a relatively fast measuring speed.
In connection with laser Doppler linear encoders (LDLE), a near-Angstrom linear encoder is a critical component for the successful development of a sub-nanometer resolution X-ray microprobe and a mirror-based X-ray interferometer. The development of a high-resolution X-ray microscope using near-Angstrom linear encoder technology could offer unprecedented capabilities for studying grain boundaries, interfaces, structures of materials and biological systems. The X-ray interferometer technology also can be valuable to calibrate the linear encoder.
The best currently available near-Angstrom linear sensor is based on the principle of capacitance measurement and has about 0.7 Angstrom resolution. However, its measuring range is only about 20 .mu.m with 2.5 mm/sec maximum measuring speed. When the measuring range is extended to the maximum of about 1.2 mm, the resolution of the capacitance sensor drops to 0.5 nm. On the other hand, the best commercial available laser interferometer system has 0.6 nm resolution with a size of 100 mm.times.100 mm.times.500 mm.
Accordingly, it is an object of the present invention to provide a new and improved compact laser Doppler encoder that is capable of measuring angular movements in the sub-nano-radian range and linear measurements in the sub-nano-meter range.
It is another object of the present invention to provide a new and improved laser Doppler encoder having very precise resolution of linear and angular rotation measurements by determining how the displacement of a target affects a laser beam that is transmitted through a self-aligning, multiple reflective optical path.
It is yet another object of the present invention to provide a new and improved laser Doppler encoder having a set of prisms on a fixed base and a set of prisms on a moving base so that the beam generated by a laser on the fixed base is reflected a substantial number of times between the prisms on the fixed and moving bases to increase the resolution of the encoder.
It is still another object of the present invention to provide a new and improved laser Doppler angular encoder for use with a high energy resolution monochromator and a laser Doppler linear encoder for use in the fields of X-ray lithography, large-field scanning microscopes, X-ray microscopes and X-ray micromachining.
Yet another object of the present invention is to provide a new and improved laser Doppler distance meter that is used with a closed loop feedback circuit to control a motion reduction mechanism to precisely position a movable base with respect to a stationary base.